Method overrides in derived classes can specify their parameters more specifically than their base class

[JakeTunaley]

TypeScript Version: 3.1.0-dev.20180912

Search Terms: class extends assignability method parameters override

Code

class Base {
    public foo (x: number | string) {}
}

class Derived extends Base {
    public foo (x: number) {}
}

const value: Base = new Derived();
value.foo('');

Expected behavior:
An error on the method definition, or an error when value is assigned, when --strictFunctionTypes is enabled.

Actual behavior:
No error when --strictFunctionTypes is enabled.

As demonstrated, this means we can pass strings to value.foo, even though Derived.prototype.foo doesn't accept strings.

This issue precludes fixing #27047, as this bug breaks the contract it relies on.

Playground Link:
http://www.typescriptlang.org/play/#src=class%20Base%0D%0A%7B%0D%0A%09public%20foo%20(x%3A%20number%20%7C%20string)%20%7B%7D%0D%0A%7D%0D%0A%0D%0Aclass%20Derived%20extends%20Base%0D%0A%7B%0D%0A%09public%20foo%20(x%3A%20number)%20%7B%7D%0D%0A%7D%0D%0A%0D%0Aconst%20v%3A%20Base%20%3D%20new%20Derived()%3B%0D%0Av.foo('')%3B

[DanielRosenwasser]

Duplicate of #5786

https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant

[JakeTunaley]

@DanielRosenwasser Sorry, I forgot to specify in the original post that this occurs when --strictFunctionTypes is enabled. I've edited the post to reflect this.

[RyanCavanaugh]

strictFunctionTypes doesn't apply to functions that were declared with method syntax